The Fabry-Perot interferometer is a high-throughput device when used for wavelength selection as a simple filter, but exhibits distinct disadvantages when used as a spectrometer. As a filter, the device can transmit a majority of the received light. Typically, more than 50 percent of the collimated incoming light can be transmitted to the next optical element.
However, when a spectrum is created using the angular wavelength selection of the device, a large amount of the incoming light is lost by reflection. This is due to the fact that wavelength selection is accomplished by reflecting the light back out of the system from other parts of the spectrum.
In a broad spectrum, the amount of light transmitted to the detector is given by the expression ##EQU1## which is the same as a set of two incoherent mirrors of reflectivity R in series. This is a serious problem when the amount of light available is limited by a discrete source. For instance, when R=0.9, which is a typical value for high resolution, the transmitted energy is only 5.3 percent of the energy available. The rest of the light is reflected back out of the system.
U.S. Pat. No. 5,666,195 to Shultz et al, entitled EFFICIENT FIBER COUPLING OF LIGHT TO INTERFEROMETRIC INSTRUMENTATION, recognizes that light recycling may be used in certain cases. More particularly, the '291 patent discloses a method of redirecting light to improve the transmission efficiency of wedged interferometers by positioning optical fibers to direct reflected light into positions enabling iterated transmission of the otherwise wasted light.
The teachings of the '195 patent are strictly limited to wedge-space etalons, or Fizeau etalons, wherein the mirrors internal to the device are not parallel to one another. As discussed in the '195 patent, "for the parallel-mirror etalon . . . the incident light is divergent, so that the reflected light continues to diverge and cannot be further employed. However, the wedged etalon . . . is illuminated with collimated light, so that the reflected light remains collimated but nevertheless also unutilized."
Thus, although Shultz et al. recognized that unused light may be recycled in a wedge-spaced interferometer, they failed to recognize that reflected light be reused in a Fabry-Perot etalon, which is an interferometer of the type having parallel reflective surfaces, as illustrated in FIG. 1A of the '195 patent. The need remains, therefore, for apparatus and methods whereby the efficiency of non-wedged-spaced interferometers may be enhanced by taking advantage of reflected light which would otherwise be wasted.